It has long been the practise to employ in machine tools of which lathes are but one example, rotatable members having a peripheral series of uniformly spaced elements each of which is to be brought into contact with a rotating work piece while the member is held against turning.
Each element of a member may be capable of performing a function differing from that performed by the other elements of that member or the elements may all be of the same type with each for use when the element in contact with the work piece is no longer fit for further service.
In either case, such members required indexing means and usually readjusting controls in order to effect the positioning of the selected element for proper engagement with the work piece.